Hovestar Hovestar - 4 months ago 14
Python Question

exec to add a function into a class

So I've looked at similar questions, and I've found some solutions to this, but I can't quite figure out how to do this.

What I'm trying to do is add a method to a class from a string. I can do this with the

setattr()
method, but that won't let me use
self
as an attribute in the extra method. Here's an example: (and I apologize for the variable names, I always use yolo when I'm mocking up an idea)

class what:
def __init__(self):
s = 'def yolo(self):\n\tself.extra = "Hello"\n\tprint self.extra'
exec(s)
setattr(self,"yolo",yolo)

what().yolo()


returns this:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: yolo() takes exactly 1 argument (0 given)


and if
s = 'def yolo():\n\tself.extra = "Hello"\n\tprint self.extra'

then I get this result:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 2, in yolo
NameError: global name 'self' is not defined


This essentially means that I cannot dynamically create methods for classes, which I know is bad practice and unpythonic, because the methods would be unable to access the variables that the rest of the class has access to.

I appreciate any help.

Answer

You have to bind your function to the class instance to turn it into a method. It can be done by wrapping it in types.MethodType:

import types

class what:
    def __init__(self):
        s = 'def yolo(self):\n\tself.extra = "Hello"\n\tprint self.extra'
        exec(s)
        self.yolo = types.MethodType(yolo, self)

what().yolo()

On a side note, why do you even need exec in this case? You can just as well write

import types

class what:
    def __init__(self):
        def yolo(self):
            self.extra = "Hello"
            print self.extra

        self.yolo = types.MethodType(yolo, self)

what().yolo()

Edit: for the sake of completeness, one might prefer a solution through the descriptor protocol:

class what:
    def __init__(self):
        def yolo(self):
            self.extra = "Hello"
            print self.extra

        self.yolo = yolo.__get__(self)

what().yolo()